This invention relates to methods and systems for measuring acoustic wave travel times in earth formations in the vicinity of the well borehole. More particularly, the present invention relates to techniques for measuring multiple acoustic wave component (or wave propagation mode) travel times in earth formations in the vicinity of a well borehole. The measurement methods use swept frequency transmitting techniques and cross correlation comparison techniques between the transmitted signal and received signal.
Sonic or acoustic well logging has become an important method for determining the physical characteristics of earth formations in the vicinity of a well borehole. Measurement of the acoustic compressional wave velocity or travel time between a transmitter and a receiver in a well borehole can define physical characteristics of the earth formations which are indicative of the capability of these formations to produce oil or gas. For example, a measurement of the compressional wave travel time or velocity gives a direct indication of the porosity of the formation in the vicinity of the well borehole. Such acoustic velocity or acoustic travel time measurements have therefore become practically standard for all new wells which are drilled.
In the prior art, acoustic pulse or pulsed sonic logging techniques have been used to measure the travel time or velocity of acoustic waves in the earth formations in the vicinity of a borehole. Such methods of the prior art have typically used impulse driven acoustic transmitters. An acoustic transmitter is fired impulsively or pulsed and the length of time necessary for the acoustic wave pulse generated by the transmitter to propagate from the transmitter through the earth formations in the vicinity of the borehole and back to an acoustic receiver located a spaced distance away from the transmitter is measured. By appropriately combining the measurements of acoustic wave travel time at several acoustic receivers, spaced different distances from either a single (or multiple) acoustic transmitter, then the acoustic wave travel time or sonic compressional wave velocity of propagation of the earth formation may be determined. Quite elaborate schemes and geometrical considerations for eliminating the effect on the travel time measurement of the borehole and borehole fluids have also been developed.
In more recent years, it has become desired to measure other acoustic wave mode travel times than merely compressional wave velocity. For example, in U.S. Pat. No. 4,131,875 issued Dec. 26, 1978, techniques are described for measuring the so called "late arrival" waves or Stonely waves. Similarly, other prior art techniques such as that shown in U.S. Pat. No. 3,354,983 issued Nov. 28, 1967, describes techniques for measuring acoustic shear wave velocities. In all of these techniques, an acoustic pulse is generated by the transmitter and the waveform of the acoustic signal at one or more receivers is analyzed in order to determine the velocity of compressional, shear, or Stonely waves in the vicinity of the borehole.
Pulsed acoustic techniques depend upon the amplitude detection of the arrival of acoustic waves at a receiver. Such techniques are prone to errors generated by random noise which occurs as a well logging instrument is moved through the borehole. Acoustic noise maybe generated by the instrument body, or centralizers on the instrument body, scraping along the sides of the borehole as the tool is moved therethrough.
Similarly, pulsed acoustic techniques involving pulsed acoustic transmitters for measuring shear waves or Stonely waves depend upon an elaborate interpretation of the waveform of the arriving wave at the receiver. Such interpretations are generally based on theoretical calculations made with simplified mathematical models of the earth formations in the vicinity of the borehole. If the simplified mathematical model proves to be in error, then the interpretation of the arriving waveform at the receiver may be in error and its relationship to more complicated real life geometries and conditions than taken into account in the model can lead to false interpretations of the waveform of the arriving acoustic signal.
It would be highly desirable to provide a method for measuring the travel time of various components of acoustic energy (compressional or primary wave, shear wave, Rayleigh or pseudo Rayleigh, direct (fluid) wave, extentional, and Stonely wave) in earth formations in the vicinity of a well borehole which was not dependent upon a theoretical interpretation of an arriving acoustic pulse waveform in terms of a model. The system of the present invention provides a direct measurement of the travel time of several components of acoustic energy from a transmitter to a receiver in earth formations in the vicinity of a well borehole.